In general, polyamide resin chips produced by melt polymerization or melt polycondensation contain about 10% by weight of the unreacted cyclic monomers or oligomers which are then eliminated by hot water extraction. Even after the elimination by hot water extraction, these cyclic monomers or oligomers are formed again during melt extrusion over a long period of time to, for example, produce fibers by melt spinning. These cyclic monomers or oligomers are accumulated around the nozzle tip and then undergo heat denaturation around the nozzle tip. As a result, yarns deformed, yarn breakage and yarn unevenness occur around the nozzle tip. In the production of films, cyclic monomers and oligomers are formed in the step of re-melting of chips, which brings about some problems such as film breakage and deterioration in the qualities. In addition, in the production of molded articles, the formation of cyclic monomers and oligomers causes problems of soiling molds and deteriorating the qualities.
Accordingly, attempts have been made to solve these problems in various ways, and the commonest method is to add inorganic compounds to polyamide resins. For example, JP-A-51-64017 proposes to add barium chloride, etc. in the step of the melt-spinning of a polyamide into fibers so as to thereby improve the melt spinning properties (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the chloride ion evolving from the compound employed in this method is not desirable, since it causes the corrosion of the equipment, etc. There have been known a number of other methods for improving the melt spinning properties of polyamides. For example, JP-B-55-18478 discloses a method which comprises applying a mold-releasing agent to the head so as to relieve the heat denaturation of the head, thus improving the melt spinning properties (The term "JP-B" as used herein means an "examined Japanese patent publication"). On the other hand, JP-B-55-20507 proposes a method for enhancing melt extrusion properties by adding a pyridinethiol oxide compound. However, the formation of cyclic monomers or oligomers in the step of re-melting can be hardly suppressed by these methods. Moreover, JP-A-1-213356 discloses a method in which the mechanical properties and heat radiation characteristics of a polyamide resin are improved by adding a certain amount of magnesium oxide, while JP-A-51-68651 discloses a method which comprises adding magnesium hydroxide as a flame-retardant. In these methods, however, the additives are employed in large amounts and, furthermore, the formation of cyclic monomers or oligomers in the step of re-melting cannot be suppressed. There are also proposed a nylon 6 composition containing magnesium oxide (JP-B-55-20007) and a nylon 6 composition containing a certain amount of fine particles of magnesium oxide having a specific crystalline structure (JP-B-55-27172). However, more than 1.0% by weight of cyclic monomers are formed in general, when chips of these resin compositions are molten again. Accordingly, these methods are still insufficient to solve the above-mentioned problems.
An object of the present invention is to provide a polyamide resin composition which suffers from little formation of a cyclic monomer in the steps of melt spinning, film formation, molding, etc. and can supply fibers, films and molded articles with excellent qualities.